Make the merge queue more flexible to flipping our favorite Jenkins instance

[gilles-peskine-arm]

We have two Jenkins instances (OpenCI and Internal CI), and we keep changing which one is set as a required status check on Jenkins. We also have DCO and readthedocs as required status checks on Jenkins, but those don't run on the merge queue: instead we fake them from one of the Jenkins instance. This has to be the Jenkins instance that actually runs in the merge queue.

On Thursday 2023-11-02 we flipped the required check from Internal CI to OpenCI on GitHub. But we forgot to update the Groovy code that fakes DCO and readthedocs, so for a while the merge queue only ran the Jenkins job but eventually timed out because there was no response to the DCO and readthedocs status checks. (Happened on Mbed-TLS/mbedtls#8464, Mbed-TLS/mbedtls#8462 and many more.)

The emergency fix #138 flips the merge queue fake to OpenCI.

The goal of this issue is to make our setup more robust so that we don't forget again. Can't we have both CIs fake the checks?